1. Field of the Invention
The present invention relates to an apparatus that outputs a high voltage to a charge apparatus that charges an image bearing member, and an image forming apparatus including the same.
2. Description of the Related Art
Among known image forming apparatuses, a laser beam printer will be described as an example. A laser beam printer includes a mechanism such as illustrated in FIG. 11. As illustrated in FIG. 11, in a laser printer, a photosensitive drum 101, which is an image bearing member, a semiconductor laser 102, which is a light source, a rotary polygon mirror 103, which is rotated by a scanner motor 104, and a laser beam 105 emitted from the semiconductor laser 102, the laser beam 105 scanning the photosensitive drum 101, are arranged.
The laser printer also includes a charge roller 106 for uniformly charging a surface of the photosensitive drum 101, a developing device 107 for developing an electrostatic latent image formed on the photosensitive drum 101, using toner, a transfer roller 108 for transferring the toner image developed by the developing device 107 onto a predetermined recording sheet, and fixing rollers 109 for heating and thereby fusing the toner transferred on the recording sheet.
The laser printer is also provided with a cassette sheet feed roller 110 that feeds a sheet from a cassette having a function that recognizes the size of recording sheets and sends the sheet out to a conveyance path, by means of one revolution, a manual sheet feed roller 111 that sends a sheet onto the conveyance path from a manual sheet feed slot having no function that recognizes the size of recording sheets, an optional cassette sheet feed roller 112 that sends a sheet onto the conveyance path from a detachable cassette having a function that recognizes the size of recording sheets, envelope feeder sheet feed rollers 113 that send sheets one by one to the conveyance path from a detachable envelope feeder in which only envelopes can be loaded, and conveyance rollers 114 and 115 that convey a sheet fed from a cassette.
In the laser printer, a pre-feed sensor 116 for detecting a front end and a rear end of a sheet fed from a source other than the envelope feeder, pre-transfer rollers 117 that send a conveyed sheet to the photosensitive drum 101, a top sensor 118 for synchronizing the writing (recording/printing) of an image onto the photosensitive drum 101 and the sheet conveyance for a fed sheet, and also for measuring the length in the conveyance direction of the fed sheet, a sheet output sensor 119 for detecting whether or not there is a sheet after fixing, and output rollers 120 for outputting a sheet after fixing to the outside of the printer are arranged.
The laser printer includes a flapper 121 that switches the destination of conveyance of a printed sheet (between the outside of the printer and a detachable double-side printing unit), conveyance rollers 122 for conveying a sheet conveyed to the double-side printing unit to a reverse part, a reverse sensor 123 that detects a front end/back end of the sheet conveyed to the reverse part, reverse rollers 124 for sequentially performing normal/reverse rotations to reverse the sheet and conveying the sheet to a sheet re-feed part, a sheet re-feed sensor 125 for detecting whether or not there is a sheet in the sheet re-feed part, and sheet re-feed rollers 126 for sending the sheet in the sheet re-feed part again onto the conveyance path.
FIG. 12 illustrates a block diagram of a circuit configuration of a control system for controlling such mechanism part. In FIG. 12, a printer controller 201 converts image code data sent from an external apparatus such as a host computer (not illustrated) into bit data necessary for printing in the printer, and reads and displays printer internal information. A printer engine control part 202, which is connected to the printer controller 201, controls operations of respective parts in a printer engine according to instructions from the printer controller 201, and notifies the printer controller 201 of the printer internal information. The printer engine control part 202 is connected to a sheet conveyance control part 203, a high-voltage control part 204, an optical system control part 205 and a fixing device temperature control part 207. The sheet conveyance control part 203 drives/stops the motors and rollers, etc., for recording sheet conveyance according to instructions from the printer engine control part. The high-voltage control part 204 performs control of respective high voltage outputs in the respective processes of, e.g., charge, developing and transfer, according to instructions from the printer engine control part 202. The optical system control part 205 controls driving/stopping of the scanner motor 104 and turning-on of a laser beam according to instructions from the printer engine control part 202. The fixing device temperature control part 207 adjusts the temperature of the fixing device to a temperature designated by the printer engine control part 202. The printer engine control part 202 is configured to receive signals from the sensor input part 206.
The printer engine control part 202 is connected to an option cassette control part 208, a double-side printing unit control part 209 and an envelope feeder control part 210, which are detachable from the printer engine control part 202. The option cassette control part 208 drives/stops a drive system according to an instruction from the printer engine control part 202, and notifies the printer engine control part 202 of a status of whether or not there are sheets as well as sheet size information. The double-side printing unit control part 209 performs an operation to reverse and re-feed a sheet according to an instruction from the printer engine control part 202, and notifies the printer engine control part 202 of a status of the operation. The envelope feeder control part 210 drives/stops a drive system according to an instruction from the printer engine control part 202, and notifies the printer engine control part 202 of a status of whether or not there are sheets.
FIG. 13 illustrates a schematic configuration of a charge bias application circuit. The charge bias application circuit includes a charge DC bias application circuit part 401, a voltage setting circuit part 402 capable of changing a set value according to a PWM signal, a transformer drive circuit part 403, a high voltage transformer part 404 and a feedback circuit part 405. In the feedback circuit part 405, the value of a voltage applied to a charge element is detected by means of R71, and is transferred to the voltage setting circuit part as an analog value. Based on the value, control is performed so as to apply a constant voltage to the charge member. Such technique is indicated in, for example, Japanese Patent Application Laid-Open No. H06-003932.
The voltage at which a discharge starts between the charge member (C roller) and the photosensitive drum (hereinafter referred to as “drum”), which is an element to be charged, varies depending on, e.g., the circumstance conditions and/or the drum layer thickness. Accordingly, simple application of a fixed voltage results in variations in drum potential (see FIG. 14). Furthermore, the drum sensitivity also differs depending on the circumstances and/or the drum layer thickness (charge transport layer thickness), and accordingly, simple application of a fixed amount of laser light results in variations in drum surface potential (hereinafter referred to as “VL”) after laser application (see FIG. 15). For example, as a method for correcting the variations, a memory is provided in a cartridge including a drum, e.g., bias values according to the sensitivities and/or usage of the photosensitive drum are stored in the memory, and based on such information, control is performed to provide a charge voltage, a developing voltage and a laser light amount according to the sensitivity and/or usage. However, with a further increase in print speed as well as an increase in capacity of the cartridge, the method of control based on the information in the memory in the cartridge has a limit in correcting variations of the voltage difference between Vdc and VL, which is illustrated in FIGS. 16A and 16B.
The present invention has been made in order to solve the aforementioned problem, and provides a high voltage control apparatus for forming a high-quality image by maintaining a potential on a photosensitive drum to be constant irrespective of the states of the circumstances and/or the drum layer thickness, and an image forming apparatus including the same.